The invention relates to a process and an apparatus for applying and impregnating fleece materials with viscous liquids.
A fleece material is herein to be understood in particular as meaning a structure of irregular texture, in which a hollow space structure nonuniformly distributed over the area is present. Such fleece materials can have as base material natural as well as artificially prepared fibres which are additionally stabilized by crosslinking with one another. Such structures are difficult to impregnate, in particular if the process of encapsulation is to take place very rapidly and without inclusion of air or gas.
Such a production stage is present for example in the manufacture of fibre-reinforced cellulose casings which are preferably prepared from a fleece web and viscose.
Cellulose casings are used for example in the packing of comestibles, in particular as sausage casings. In this area, a distinction is made between casings coated with viscose on one or both sides, depending on the way the application of viscose is carried out in the course of manufacture.
The form which is coated with viscose on both sides generally exhibits a better and more uniform encapsulation of the fibre fleece, provided it is possible to avoid air inclusions in the short time of application of viscose to both sides of the fleece. It will be readily understood that the controlled displacement of air is easier to carry out in the case of a one-sided coating, in particular since, after leaving the die in which the coating is applied, there is still a free section, to the start of coagulation, which permits the one-sided displacement of air from the fleece.
On the other hand, there are areas where only fleece-reinforced cellulose casings coated with viscose on both sides are sued in practice. They are in general the fibre skins which have been lacquered on the inside with a barrier layer. The inner surface of the casing has a cellulose layer for reasons of lacquerability, while the outer surface has a cellulose covering for visual reasons. The customarily dope-dyed regenerated cellulose present on the outside provides optimal cover for the fibrous structure. This type of casings is widely used in the making of scalded and boiled sausages.
It has further been found that even in the case of unlaquered material there are in many areas significant advantages on the side of using material coated on both sides with viscose. For instance, in the event of high internal friction in the filling process the sliding behaviour with casings having an inside layer of cellulose is significantly more favourable on account of the excellent surface smoothness. Such is the case with uncooked sausage meat fillings or with skins for covering hams. Additional sliding impregnations which are necessary in the case of one-sided application of viscose to the outside can be dispensed with. The application of viscose to both sides also ensures a more uniform overall structure which is valued on account of the excellent peeling behaviour and the more uniform dilation of the casing in particular in the case of cold cuts which are packaged a second time.
These examples show the importance which two-sided coating with viscose already has and which will be increased if it is possible, in particular, to obtain the manufacturing advantages of one-sided coating with viscose. The two-sidedly viscose-coated cellulose casings in tube form which are customary today are manufactured, in general, by first forming a web of fibre fleece into a tube. This tube is coated on both sides with viscose in the coating means (GB No. 1336850). The viscose is applied almost simultaneously in this case because of the decrease in strength on wetting the customarily preferably used natural fibre fleece with viscose. Coating is effected with die systems which consist of an outer and an inner annular die. The viscose is applied under pressure via these annular dies in predetermined amounts. The reason it is difficult to impregnate the fleece without including air is that highly viscous aqueous liquids are concerned here.
The viscosity of the viscose is essentially determined by the solids content in terms of cellulose and the degree of polymerization. The higher these two values, the higher the viscosity. To process viscose, a low viscosity is desirable for rapid and optimal impregnation of the fibre fleece. However, the quality of the end product grows with a high solids content and degree of polymerization. The degree of polymerization determines the shrinkage behaviour and the elasticity of the regenerated cellulose, while the solids content determines the porosity and, in conjunction with the degree of polymerization, the final strength of the casting. The manufactured article is consequently a comprise which usually has a solids content which is actually too low.
To avoid the difficulties in the case of the twosided coating with viscose, the viscoses used have a solids content of 6.5 to 7% by weight, that is a solids content which is not optimal.